Frankly, I think you would be better off looking at a new rig. Your motherboard is very limited, especially with regards to cpus, memory and drive compatibility. If you start upgrading, you will almost certainly want a new mobo/cpu and this would require you to purchase a new copy of windows as your current license is married to the motherboard. Also, Windows XP will officially be out of support shortly, and unless you have a slipstreamed dvd of all updates, etc., you will be hard pressed to repair the os in case of failure.
If you really want to try casual gaming/streaming, you can try a lightweight video card like a GT640 or Radeon 7750, but with the slower processor and memory you have, it might be disappointing. In addition, your power supply is also suspect, as it is now 8 years old and is not known for being efficient. IMO, you would really be better off getting a new rig, or building a budget rig yourself.
Mark